1. Field of the Invention
The invention relates to a shadow mask to be used for a cathode ray tube, having a plurality of through-holes, such as dot holes and slot holes, each of which is defined by a greater-size recess formed at a first surface thereof and a smaller-size recess formed at a second surface thereof. The invention further relates to a method of fabricating the shadow mask, and still further to a cathode ray tube including the shadow mask.
2. Description of the Related Art
One of conventional color cathode ray tubes has been suggested in Japanese Unexamined Patent Publication No. 7-65738. FIG. 1 illustrates the suggested color cathode ray tube. The illustrated color cathode ray tube 11 includes a bulb 12 having a face panel 13 constituting a front surface of the bulb 12, and a neck portion 12a, a fluorescent film 14 formed on an inner surface of the face panel 13, a shadow mask 15 disposed in facing relation with the fluorescent film 14 and having a plurality of slots, an electron gun 16 disposed in the neck portion 12a of the bulb 12, and a deflecting yoke 18 disposed around the neck portion 12a of the bulb 12 for deflecting electron beams 7 emitted from the electron gun 16.
In operation, the electron gun 16 emits the electron beam 7, which is deflected by a magnetic field generated by the deflecting yoke 18. The deflected electron beam 7 passes through the shadow mask 15, and scans the fluorescent film 14 therewith. In accordance with the scanning path, a certain image is produced on the fluorescent film 14.
In order to enhance basic characteristics expected in an image display device, such as contrast and brightness, the color cathode ray tube is designed to include, on an inner surface of the face panel 13, a black matrix film (not illustrated) comprising non-luminous light-absorbing material, such as black carbon, filling spaces formed between red, green and blue fluorescent luminous pixels, and a metal back film (not illustrated) which is made of an aluminum film and which reflects light independently of the fluorescent film 14. The above-mentioned fluorescent film 14 is integrally formed with the black matrix film. The shadow mask 15 is disposed in facing relation with the metal back film.
Hereinbelow is explained the shadow mask 15 having a plurality of rectangular slots through which the electron beam 7 passes.
As illustrated in FIG. 2, the shadow mask 15 is formed with a plurality of slots 22 each of which has a longer side in a direction of a vertical axis V and a shorter side in a direction of a horizontal axis H. Bridge portions 23 are formed between the adjacent slots 22 in the vertical axis V direction, and connecting portions 24 are formed between the adjacent slots 22 in the horizontal axis H direction.
Each of the slots 22 is a through-hole comprised of a first recess 25 formed at a first surface of the shadow mask 15, and a second recess 26 formed at a second surface (not seen in FIG. 2) of the shadow mask 15 and having a smaller size than the first recess 25. Herein, the first surface of the shadow mask 15 is defined as a surface facing the fluorescent film 14, and the second surface is defined as a surface facing the electron gun 16. The slots 22 are formed by the steps of forming a first photoresist pattern on a first surface of a thin metal plate for forming the first recess 25, which first photoresist pattern defines a plurality of rectangles each of which has a longer side in the vertical axis V direction and a shorter side in the horizontal axis H direction, forming a second photoresist pattern on a second surface of the thin metal plate for forming the second recess 26, which second photoresist pattern also defines a plurality of rectangles each of which has a longer side in the vertical axis V direction and a shorter side in the horizontal axis H direction where the longer and shorter sides in the second photoresist pattern are shorter than those in the first photoresist pattern, etching the thin metal plate with the first and second photoresist patterns acting as a mask to thereby form the first and second recesses 25 and 26, and removing the first and second photoresist patterns.
FIG. 3 is a cross-sectional view taken along the line III--III in FIG. 2, illustrating a positional relation between the slot 22 and the incident electron beam 7 passing through the slot 22. As illustrated in FIG. 3, if the electron beam 7 partially strikes an inner surface 26a of the second recess 26, a part of the electron beam 7 is randomly reflected in a direction different from a direction in which the electron beam 7 is originally directed. If the randomly reflected electron beam 7a was directed towards the fluorescent film 14, an undesired image would be generated on the fluorescent film 14 by the randomly reflected electron beam 7a, which is a major factor for degrading the contrast of the shadow mask 15.
The electron beam 7 enters, at a greater incident angle, the slot 22 located farther away from a center of the shadow mask 15, and accordingly, is reflected at the inner surface 26a of the second recess 26 to greater degree, resulting in that the contrast of the shadow mask 15 is considerably degraded.